Methods of achieving organ substitution involve three broad classes of approaches. In the first, replacement organs are realized by the construction of electromechanical devices, such as the recently developed, wholly implantable mechanical replacement heart. A second alternative involves the use of xenotransplantation, or animal organs, rather than human donor organs. The third general category of providing replacement function for tissues and organs involves the rapidly emerging field of tissue engineering.
The principal disadvantages of mechanical devices and xenotransplantation involve the challenges of integrating these devices and tissues within the host. In the former case, mechanical devices utilize materials that are foreign to the host and therefore engender processes such as inflammation and clotting. Additionally, mechanical devices are inherently temporary in nature, since they require artificial power supplies, control circuitry and other features that can never fully integrate into a natural system. In the case of xenotransplantation, the human immune system is designed to reject cells and tissues from foreign species and therefore, immune system suppression and its inherent risks remain a challenge. Additional risks involve the transmission of genetic viruses from the donor animal to the host.
Tissue engineers have taken several approaches to generate replacement tissues and organs in the laboratory. Generally, autologous tissue (from cells taken from the organ recipient) are seeded onto a scaffold and expanded in culture. This scaffold must be biologically compatible to avoid inflammatory responses and rejection of the implanted device, and may be biodegradable so that the artificial material bioresorbs, leaving only natural tissue. Scaffold fabrication techniques include an array of polymer processing techniques such as molding, casting, fiber mesh fabrication, and solid freeform fabrication. All of these methods lack the resolution necessary to fashion the finest features of the organ, such as the capillaries, which predominate the circulatory supply. More recent developments in microfabrication technology, such as MEMS (MicroElectroMechanical Systems), which includes silicon micromachining and polymer replica molding, have improved construction of artificial tissue and organ scaffolds. Typically, the resolution of these techniques is in the 10 nm-1-micron range, well in the range of what is necessary to configure the highest fidelity features of an organ.
Within the field of tissue engineering, two basic methods for organizing cells into tissue structures and organs are being pursued. For most tissues and organs, multiple cell types are required; the most significant requirement in addition to the need for replacement of specific organ function is the cellular component of the vascular supply, which nourishes the tissue. One such class of methods utilizes biochemical factors, chemical gradients, growth factors and other chemical means to arrange multiple cell types on a substrate. These chemical techniques may involve the micropatterning of the scaffold surface for cell and tissue engineering with surface chemistries, which enhance adhesion, growth, alignment and other behaviors of specific cell types.
The second broad class of methods utilizes precision loading of specific cell types into separate microengineered compartments within the tissue-engineered structure. Such an approach often invokes microfluidic loading, either dynamically or statically, of a network of channels or vessels connected to form a cell compartment, with a semi-permeable barrier that physically separates cells from all other compartments. In sequential fashion, each compartment of a bilayer structure is loaded with the specific cell type, and communication between compartments is controlled by porous or non-porous barriers (J. T. Borenstein, et al. Proc. 12th Int'l. Conf. Solid State Sensors, Actuators and Microsystems (Transducers 2003), 1754-7 (2003)). Properties of the barrier material are governed by the requirements of the specific cells and tissues in adjacent compartments. For instance, in the case of organ-specific cells such as hepatocytes contained in a compartment adjacent to the endothelial cells comprising the vascular supply, the barrier material, or membrane, must physically separate the cell types from adjacent compartments during cell seeding, but must readily enable the transfer of oxygen, nutrients and waste products between the two compartments.
This microfabrication process is inherently two-dimensional in nature, and one way to construct a three-dimensional tissue engineered device is to stack multiple layers of cell sheets, with microfabricated membranes interspersed between these layers. The interspersed membranes contain pores that govern the transport properties of the film; the pore size, porosity and permeability of the membrane are controlled to provide the appropriate behavior for the desired application. The stacking process proceeds as follows: a microfabricated polymer film imprinted with a pattern for the compartment containing organ-specific cells, such as hepatocytes, is placed at the bottom of the stack. Next, a membrane with controlled transport properties is placed over the compartment, and bonded to the compartment layer. Next, a microfabricated polymer film with vascular channels imprinted (face down) is placed over the membrane, and bonded to it. In this manner, cells loaded into the lower compartment communicate with the vascular channels located above it by transport through the membrane. Three-dimensional integration of this assembly is achieved by situating vertical through-holes within each of the layers, thus forming “pipes” along the z-axis. Pipes for the organ-specific compartment are connected laterally to the compartments within each organ-specific layer, but are separated from the pipes associated with integration of the vascular layers.
One of the principal drawbacks of the foregoing approach involves the restrictions on the design of organ structures. For instance, the stacking approach described above results in capillary beds that are oriented laterally within the polymer film containing the vascular network, but does not allow for vertically oriented blood vessels other than the inflow and outflow pipes integrating the entire network. This restriction results in a limited density of capillaries within the three-dimensional structure; the ratio of small blood vessels to large vessels is not nearly as high as it is in physiological systems. Thus, there remains a need in the art for devices and methods that can replicate the requisite features of the organ it is replacing, such as the fluid dynamics of the vascular supply and other organ structures.